1. Field of the Invention
The present invention relates to an electric rotary machine (hereinafter referred to occasionally as xe2x80x9cmotorxe2x80x9d as appropriate), and particularly to a radial gap type electric rotary machine having a structure of divided salient poles.
2. Description of the Related Art
There has been a conventional electric rotary machine including a stator armature (herein after referred to as xe2x80x9carmaturexe2x80x9d) structured such that a plurality of ring-shaped yoke pieces, which are made of soft magnetic steel plates (silicon steel plates, etc.) and which each have a plurality of pole tooth portions radially protruding, are stacked in the axial direction. When manufacturing this conventional electric rotary machine, each of the ring-shaped yoke pieces is punched out integrally with the plurality of pole tooth portions as a single piece. Therefore, the pole tooth portions are not structurally separated from the yoke piece and a resultant armature has a superior magnetic efficiency (low magnetic reluctance).
However, in case of a small electric rotary machine, since a wire is usually wound directly on each pole tooth portion, the integrally structured armature makes the winding operation troublesome. In particular, in case of an inner rotor type rotary machine, the winding operation is extremely difficult. As a result, there are two problems in that the winding operation requires a long time, and that it is impossible to increase the space factor of the wiring. And also, because the winding is done by flyer winding, torsional stress may be applied to a wire during the winding operation to restrict insulation reliability of the winding portion.
Confronted with these difficulties, in recent years, a rare earth magnet having high energy product has been developed and the structure of the electric rotary machine has been reviewed by the analysis of magnetic circuits using a computer. This results in enabling a divided armature type electric rotary machine to achieve desired motor characteristics. Although magnetic reluctance is slightly increased in this divided armature type electric rotary machine, the winding operation is easy and also the space factor can be increased, which outweighs the undesirable increase in magnetic reluctance.
As a result, it has been recently found that the divided armature type electric rotary machine is superior on the whole in the achievement of higher performance and lower cost. Therefore, nowadays there is a growing demand for the divided armature.
One example of the above divided armature is conventionally manufactured such that an armature yoke which is constituted by a plurality of ring-shaped plate yoke pieces stacked in their axial direction has its pole tooth portions separated from its main body portion and a winding is provided around each of the pole tooth portions separated, and then, the pole tooth portions with respective windings therearound, namely salient poles, are joined back to their respective original locations on the main body portion by laser beam welding or the like.
However, this conventional method needs much labor because the armature is once divided and then joined again. Moreover, when the separated pole tooth portions are put back together, the stack layers of the both portions have to be properly matched with each other. Therefore, it is necessary to set the divided pole tooth portions in a well maintained die or tool which ensures the assembling precision satisfactorily and also to securely weld them on a one-by-one basis for reinforcement. As a result, it is difficult to maintain high precision, and workability is low. In addition, the mechanical and magnetic characteristics are significantly degraded at the joined (welded) portions inherently. Thus, the conventional divided type armature still has a few problems to be solved.
In order to solve such problems, the present inventors have proposed in Japanese Unexamined Patent Publication No. 2001-238377 a radial gap type electric rotary machine with a divided armature comprising a plurality of separated pole tooth portions and two kinds of rings which magnetically and mechanically join the separated pole tooth portions. One ring is a pole tooth ring for positioning and fixing the pole tooth portions and the other ring is a stator ring for reducing the leakage flux resulting from magnetic discontinuity.
FIG. 1 is a partially sectional front elevation view seen from the axial direction showing the electric rotary machine which has been disclosed in Japanese Unexamined Patent Publication No. 2001-238377. FIG. 1 shows a rotor assembly 120, salient poles 106, pole teeth 134 each constituting each salient pole 106, resin spaces 204 each provided between one salient pole and its adjacent salient pole, and filled with resin at the end of assembling the electric rotary machine, and a stator ring 200 to form a magnetic circuit by enclosing all the salient poles 106.
FIG. 2 is an enlarged view of a contact portion between the pole tooth 134 and the stator ring 200 in the electric rotary machine shown in FIG. 1. FIG. 2 shows a front end surface 134a of the pole tooth 134, an inner surface 201 of the stator ring 200, and an air gap 202 formed between the inner surface 201 and the front end surface 134a. 
However, the following problems arise in the conventional electric rotary machine disclosed in the above Japanese Unexamined Patent Publication No. 2001-238377.
As shown in FIGS. 1 and 2, the front end surface 134a of the pole tooth 134 opposite to an end surface facing the rotor assembly is flat, while the inner surface 201 of the stator ring 200 is arced since the stator ring 200 is cylindrical.
Therefore, when the pole tooth 134 is joined to the inner surface 201 of the stator ring 200, only both very edges of the front end surface 134a of the pole tooth 134 are in contact with the inner surface 201 of the stator ring 200, thus generating air gap 202 between the front end surface 134a of the pole tooth 134 and the inner surface 201 of the stator ring 200. Therefore, this structure causes loss in a flux leakage passing from the pole tooth 134 of the salient pole 106 to the stator ring 200 thereby decreasing magnetic efficiency.
The decrease in magnetic efficiency will become sharp as the curvature of the stator ring 200 becomes small, and it will deteriorate the motor characteristics. This gives a serious problem in the development of the electric rotary machine having a small diameter for downsizing.
Furthermore, in the above conventional manufacturing method the pole tooth of each salient pole is in contact with the stator ring along lines, and even at points in the extreme case depending on the variation in precision of assembly. Therefore, it is extremely difficult to maintain a stable and uniform contact therebetween.
The above unstable contact between the pole teeth of the salient poles and the stator ring causes uneven magnetic reluctance between the salient poles and the stator ring, and as a result, the electric rotary machine loses its magnetic balance on the whole, which will cause the torque ripple to increase, thus making the electric rotary machine vibrate easily while rotating. For example, when the electric rotary machine with this problem is used for a printer, print-density may be incurred.
The present invention has been made in view of the above-described circumstances in the prior arts, and it is an object of the present invention to provide a radial gap type electric rotary machine, wherein a stator ring and a plurality of salient poles are in contact with each other without any air gap therebetween so that stable motor characteristics can be obtained.
In order to achieve the above object, according to a first aspect of the present invention, an electric rotary machine includes a stator which comprises: a plurality of salient poles each including a pole tooth; and a stator ring adapted to enclose the salient poles thereby forming a magnetic circuit, the stator ring having its inner surface formed flat at portions each to be jointed to an outer end face of the pole tooth thereby realizing surface-to-surface contact between the stator ring and the pole teeth.
According to a second aspect of the present invention, in the electric rotary machine of the first aspect, the stator ring is tubular with a polygonal cross-section.
According to a third aspect of the present invention, the electric rotary machine of the second aspect further comprises a pole tooth ring, which is adapted to enclose, position and fixedly hold the above salient poles, is disposed inside the stator ring, and which is tubular with a polygonal cross-section.
According to a fourth aspect of the present invention, in the electric rotary machine of the second or third aspect, the cross-section is dodecagonal.
According to a fifth aspect of the present invention, in the electric rotary machine of the second aspect, the stator ring is formed such that both ends of a flat steel plate are joined to each other at one of the portions formed flat.
According to a sixth aspect of the present invention, in the electric rotary machine of the third aspect, at least one air gap is provided between the pole tooth ring and the stator ring.
According to a seventh aspect of the present invention, in the electric rotary machine of the third aspect, the stator ring and the pole tooth ring are joined to each other such that one end of a flat steel plate for the stator ring is joined to the other end of a flat steel plate for the pole tooth ring and the other end of the flat steel plate for the stator ring is joined to one end of the flat steel plate for the pole tooth ring, whereby the stator ring and the pole tooth ring are both formed into a tubular configuration and at the same time are joined to each other.
According to the present invention, a magnetically well-balanced contact is realized between the stator ring and the salient poles, thereby forming a stable magnetic circuit, which results in suppressing the vibration of a motor in rotation. Also, the stator ring and the pole tooth ring can be assembled with reduced difficulty, which results in easily rendering the magnetic connection stable and in reducing the manufacturing time.